1. Field of the Invention
The present invention relates to a feed horn for satellite broadcasting reception. Particularly, the invention is concerned with a feed horn suitable for a parabola antenna having an elliptic reflector with the major axis of the ellipse extending in the horizontal direction.
2. Description of the Prior Art
A conventional feed horn will now be described with reference to FIGS. 6 to 8. The feed horn is used in a parabola antenna 41 for the reception of satellite broadcasting signals. The receiving antenna 41 is provided with a circular reflector 42, the feed horn indicated at 43, and a reception circuit (not shown). The reflector 42 and the feed horn 43 are opposed to each other. The reflector 42 reflects a signal transmitted from a broadcasting satellite and the reflected wave from the reflector 42 is inputted to the reception circuit through the feed horn 43.
The feed horn 43 has a horn portion 44 and a cylindrical waveguide 45 connected to the horn portion 44. The horn portion 44 is provided for making it easier to pick up the reflected wave from the reflector 42 in the reception antenna 41. The horn portion 44 has an opening 46 of a circular section which is formed in a divergent horn shape from one axial end 44a of the horn portion toward a front end 44b.
The waveguide 45 is a transmission path for conducting a transmitted signal to the reception circuit, with an opening portion 45a of a circular section being formed in the interior of the waveguide. One end 45b of the opening 45a in the axial direction of the waveguide 45 and the axial end 44a of the opening portion 46 are connected together and an opposite end 45c in the axial direction of the waveguide 45 is connected to the reception circuit.
In the feed horn 43 constructed as above, a signal transmitted from a broadcasting satellite is reflected by the reflector 42 of the receiving antenna 41 and the reflected wave is inputted to the reception circuit via the feed horn 43.
With the recent tendency to a multi-channel satellite broadcasting, a large number of broadcasting satellites lie on the geostationary orbit and there is now a demand for improvement in directivity in the longitude direction of the receiving antenna so that a reception side of a satellite broadcasting can receive a signal transmitted from only a broadcasting satellite that transmits a desired broadcasting program from among such many broadcasting satellites.
However, in the use of the receiving antenna 41 constituted of both the circular reflector 42 and the feed horn 43 possessing a circular directivity, there is no other method than increasing the area of the circular reflector 42 for improving the directivity, thus giving rise to the problem that the cost of the antenna becomes high.
Moreover, since the feed horn 43 has the opening portion 46 of a circular section constituted of one side face portion which expands in the shape of a horn from one axial end 44a toward the front end 44b of the horn portion 44, the transmitted signal is apt to be reflected at the side face of the opening portion 46. Although this reflection can be diminished by making the inclination of the inner side face of the opening portion 46 steep, it is necessary that the distance D5 between the one axial end 44a and the front end 44b of the horn portion 44 be set long. As a result, not only the feed horn 43 becomes longer and larger in size, but also the amount of the material required for fabricating the feed horn 43 increases, thus giving rise to the problem that the cost of the feed horn 43 becomes high. Further, crosstalk is apt to occur because the electromagnetic field distribution in the opening portion 46 of the feed horn 43 is an H.sub.11 mode distribution.
The use of an elliptic reflector may be effective as a method for reducing the area of the circular reflector 42 and improving the directivity in the longitude direction. However, if the feed horn 43 of a circular directivity shown in FIGS. 7 and 8 is combined with an elliptic reflector, the directivity of the feed horn 43 no longer matches the shape of the reflector, so that not only the reception efficiency is deteriorated but also it becomes easier to pick up noise signals from the surroundings, thus leading to deteriorated directivity of the antenna.